Optical fiber waveguides operate by the principle of total internal reflection which requires that the refractive index of the core, or inner portion of the waveguide be greater than the refractive index of the clad or outer portion. The actual variation of the refractive index at the core-clad region determines the waveguide dispersion. Mathematical formulations of waveguide dispersion have been presented and show that, for communication systems, graded index profiles resembling parabolae with the maximum at the fiber axis minimize dispersion. Denoting index of refraction by n and radial distance from the preform or fiber axis by r, the desired variation in index for optical communications is generally written as: EQU n=n.sub.0 (1-ar.sup..alpha.)
where n.sub.0 is the maximum index at the fiber axis, a and .alpha. are design constant determined by the desired numerical aperture of the optical waveguide and its proposed use. The exponent .alpha. generally varies between 1.8 to 2.5. In the design of other integrated optics applications, such as for endoscope lenses or elements and other types of image transfer devices, it is often useful to use a more complex relation between n and r, which represents the next most useful equation: EQU n=n.sub.0 (1-ar.sup..alpha. +br.sup..gamma.)
The constants in this equation are based on the desired optical performance design consideration and characteristics and can generally be derived by one practiced in the art. These equations give a mathematical description of the desired shape of the refractive index variation in the fibers, lenses and integrated optic elements.
A number of methods have been disclosed for producing graded refractive index profiles in fibers, lenses and integrated optics elements. One method for producing graded refractive index profiles in glass employs the so-called process of molecular stuffing and is disclosed in U.S. Pat. Nos. 4,110,093 and 4,110,096 issued Aug. 29, 1978. This method uses porous glass preforms whose pores are completely filled with a solution (called a stuffing solution) containing one or several index modifying dopants which raise the refractive index of the material making up the porous glass preform. The variation in refractive index is produced by soaking the filled preform in a solvent solution (called an unstuffing solution) which is free of dopant and has a solubility limit for the dopant which is higher than the dopant concentration in the solution within the pores. This allows dopant to diffuse back into the solvent and therefore produces a depleted region in the preform where the concentration of the dopant varies continuously from minimum to a maximum at some depth within the preform.
The above mentioned single step diffusion process limits the type of profiles obtainable and cannot sufficiently approach the desired parabolic profile. Satisfactory profiles have been obtained with the above process when:
(a) After stuffing, the preforms are soaked in 100.degree. C. water for a time to be determined by experiment (15-45 min.) which varies depending upon the pore size of the porous glass used.
(b) The preform is immersed in 0.degree. C. propanol until precipitation occurs.
Good parabolic profiles are reliably and reproducibly obtained by the above method if:
(1) Sample test runs are conducted in advance to determine the appropriate combination of unstuffing and precipitation times, and
(2) Rods from glass having identical pore size are used. It will be understand that pore size depends on glass composition and heat treatment, more particularly heating temperature and annealing time.
A slight improvement in the quality of the profile, and a reduction of the variability of times has been obtained by the following process when:
(a) After stuffing, the preforms are soaked in 100.degree. C. water for about 10 minutes.
(b) The preforms are then soaked in a 40% ethanol-water solution at 70.degree. C. for a time to be determined by experiment (15-60 min.).
(c) The preforms are immersed in 0.degree. C. propanol until precipitation occurs.
The above modified method yields slightly higher numerical apertures, but both last mentioned processes lack sufficient control over the experimental parameters, and are too sensitive to the processing times. In both cases, the times to precipitation vary with the soaking times. In addition, for long preforms normally of a length of 30 cms to 1 meter, precipitation times will be different at different ends of the preforms, possibly leading to a variation in profile along the length of the preform.